H 

£.11 

97     f 

CHOCKED 
CASE 


BIOLOGY 
LIBRARY 


HOW  TO  USE  AND 
CARE  FOR  THE 

MICROSCOPE 


COMPLIMENTS  OF 

SPENCER  LENS  CO. 

BUFFALO,  JV.  Y. 


How  TO  USE  AND  CARE  FOR 
THE 

MICROSCOPE 


A  Simple  Treatise  on  the  Use  of  the  Microscope 
especially  adapted  to  laboratory  work. 


Published  by 
SPENCER    LENS    CO. 

BUFFALO.    N.   Y. 


__  ..Ocular, 


BIOLOGY 
LIBRARY 


rse  Shoe  Base, 


PREFACE 


A  microscope, — like  any  other  instrument  of  precision, 
— cannot  be  made  to  do  its  best  work  without  some 
knowledge  on  the  part  of  the  user  as  to  .how  to  keep 
it  in  perfect  condition,  and  an  intelligent  understanding 
of  its  parts,  their  use  and  relations. 

The  purpose  of  this  little  book  is  to  bring  before  you 
some  practical  helps  in  clear  concise  form,  which  will 
enable  you  to  obtain  the  best  possible  results  when  work- 
ing with  the  microscope. 

We  make  no  attempt  to  go  into  the  optical  principles 
involved  in  the  microscope  except  in  so  far  as  it  is  neces- 
sary to  direct  in  its  use.  For  these  we  refer  you  to  more 
pretentious  works, — The  Microscope,  by  Dr. S.H.  Gage; 
The  Microscope  and  its  Revelations,  by  Carpenter  & 
Dallinger. 

Let  it  be  remembered  that  no  amount  of  direction  will 
take  the  place  of  good  judgment  and  careful  painstaking 
effort  on  your  part,  and  that  it  is  only  the  perfect  ad- 
justment of  every  part  in  relation  to  every  other  part 
which  brings  the  best  results.  The  neglect  of  one  detail 
may  destroy  the  virtues  of  all  the  others. 

It  is  with  a  desire  to  help  and  a  hope  that  your  work 
with  the  microscope  may  prove  pleasant  and  profitable 
that  we  present  this  little  volume. 

SPENCER   LENS   CO. 

266119 


DONT'S 

Don't  allow  dust  and  dirt  to  settle  on  the  microscope. 

Don't  carry  the  microscope  by  the  arm. 

Don't  use  alcohol  on  the  microscope. 

Don't  expect  too -great  a  range  in  the  fine  adjustment. 

Don't  take  the  fine  adjustment  apart. 

Don  t  bring  the  objective  into  contact  with  the  cover 
glass. 

Don't  fail  to  focus  up  before  turning  the  nosepiece 
unless  you  know  the  objectives  are  parfocal. 

Don't  forget  that  high  powers  have  short  working  dis- 
tances. 

Don't  focus  down  with  the  eye  at  the  eyepiece. 

Don't  fail  te  secure  good,  even  illumination. 

Don't  drop  the  objectives  and  oculars. 

Don't  try  to  take  an  objective  apart. 

Don't  try  to  work  with  dirty  lenses. 

Don't  try  to  clean  them  with  a  dirty  cloth. 

Don't  fail  to  clean  oil  from  an  immersion  lens  im- 
mediately after  using. 

Don't  try  to  work  with  an  immersion  lens  when  there 
are  air  bubbles  in  the  oil. 

Don't  use  high  powers  when  low  ones  will  do. 

Don't  use  higher  oculars  than  necessary. 

Don't  expect  a  lens  to  work  at  its  r/est  unless  used  on 
a  cover  thickness,  and  with  a  tube  length,  for  which  it 
is  corrected. 

Don't  shut  one  eye. 

Don't  get  discouraged  if  desired  results  do  not  come 
immediately. 

4 


"ube  Diaphragm 
with  Society  Screw 


the 

..'JFronUens     \0bjecl  i  ve 
..WorKingD'fttanc*/ 


Slide' 


Object 


FIG.  1.    AUL  OF  THE  PARTS  MENTIONED  IN 
THE  TEXT  ARE  INDICATED  ON  THIS  FIG- 
URE AND  THE  FRONTISPIECE. 


PART  I 

CARE  OF 

THE 
MICROSCOPE 

THE  STAND 

Every  microscope 
regularly  leaves  the 
factory  in  a  case 
made  especially  for 
it  in  which  it  fits 
securely.  If  it  is  to 
be  carried  any  dis- 
tance it  should  be 
carried  in  the  case. 
It  should  be  left  in 
the  case  when  not 
in  use  unless  some 
other  means  are  pro- 
vided to  protect  it 
from  dust. 

Dust  settling  upon 
the  highly  polished 
surfaces  is  apt  to 
scratch  them  when 
it  is  removed.  It 
works  into  the  bear- 
ings of  the  instru- 
ment, making  them 
work  hard  and  un- 
necessarily wearing 
them. 


The  microscope  ought  not  to  stand  continually  ex- 
posed to  direct  sunlight. 

When  removing  the  microscope  from  the  case,  or  in 
handling  it  in  the  laboratory,  do  not  grasp  it  by  the  arm 
(see  frontispiece),  which  is  actuated  by  the  delicate  fine 
adjustment.  Grasp  it  by  the  pillar  below  the  stage  or 
by  the  stage  just  where  the  pillar  joins  it.  The  best 
modern  instruments  are  provided  with  a  new  fine  ad- 
justment at  the  side,  permitting  the  introduction  of  a 
handle  by  which  the  microscope  can  be  easily  handled. 
All  rough  handling  is  so  out  of  harmony  with  the.  use  of 
such  a  delicate  instrument  that  it  is  unnecessary  to 
caution  against  it. 

Finger  marks  on  the  highly  polished  surfaces  should 
be  removed  immediately  with  a  soft  cloth  or  clean  cham- 
ois skin.  If  they  do  not  remove  easily,  breathe  on  the 
surfaces  and  rub  gently.  If  allowed  to  remain  for  any 
time  they  are  removed  with  great  difficulty.  If  the  sur- 
faces become  soiled  with  material  which  gentle  rubbing 
will  not  remove,  dampen  a  cloth  with  water  and  rub 
gently.  If  this  will  not  remove  it  use  a  very  little 
xylol,  ether  or  chloroform,  being  careful  not  to  rub  too 
hard,  and  to  dry  as  soon  as  possible.  Never  use  alcohol 
on  lacquered  parts.  It  will  dissolve  the  lacquer, — no 
matter  how  dilute  it  is.  When  the  lacquer  is  gone 
nothing  can  be  done  for  it  outside  of  the  factory.  All 
reagents  should  be  kept  from  the  lacquer  wherever  pos- 
sible. On  most  of  the  better  modern  microscopes  the 
more  exposed  parts  are  so  finished  that  they  are  not 
seriously  affected  by  any  of  the  above  mentioned  rea- 
gents, with  the  exception  of  the  strong  acids.  All  such 
reagents  should  be  removed  as  soon  as  possible.  This 
finish  is  generally  a  black  enamel  or  a  gray  or  black 
plating.  Sometimes  this  finish  is  extended  to  the  upper 
parts  of  the  stand. 


Stage.  On  account  of  its  exposed  position  the  finish 
of  the  stage  is  worthy  of  special  mention.  The  stages 
of  the  cheaper  microscopes  are  finished  with  a  prepara- 
tion which  gives  the  brass  a  dull  black  appearance. 
They  are  easily  cleaned  as  directed  above.  When  they 
become  gray  and  dingy  a  very  little  of  one  of  the  heavier 
oils  rubbed  upon  them  will  often  make  them  black  again, 
unless  the  finish  be  worn  off.  The  stages  of  all  the 
better  microscopes  are  covered  with  hard  rubber  which 
is  not  permanently  effected  by  any  of  the  ordinary 
laboratory  reagents.  Should  the  stage  become  soiled 
with  balsam,  immersion  oil,  or  anything  which  water 
will  not  remove,  it  can  be  cleaned  with  xylol  or  chloro- 
form. The  xylol  will  turn  the  black  stage  to  a  dull 
gray,  but  a  little  of  some  heavy  oil  rubbed  upon  it  will 
restore  the  original  black.  If  the  gray  color  is  of  long 
standing  it  may  be  necessary  to  leave  the  oil  on  some 
time.  Wipe  off  the  oil  thoroughly  when  it  has  done  its 
work. 

Inclination  Joint.  All  of  the  better  microscopes 
are  provided  with  the  inclination  joint  by  which  the 
body  can  be  inclined  to  any  angle  between  perpendicular 
and  horizontal.  Once  in  a  while  this  joint  wears  loose 
so  that  the  microscope  will  not  remain  at  the  desired 
angle.  This  can  be  tightened  by  tightening  the  nuts 
on  the  ends  of  the  inclination  axis  with  a  heavy  screw 
driver  if  the  nut  is  slotted,  or  with  a  "spanner"  if  the 
nut  it  provided  with  two  small  holes.  A  pair  of  round 
nosed  pliers  will  serve  the  purpose  nicely  in  the  absence 
of  the  spanner.  With  any  of  the  tools  great  care  should 
be  taken  not  to  mar  the  nuts.  On  most  of  the  modern 
instruments  the  pin  which  forms  the  axis  is  slightly 
conical  and  the  necessary  friction  is  obtained  by  drawing 
the  cone  tighter  into  its  bearings.  This  necessitates  the 
7 


secure  the  pin  and  put  it  in  place.     This  ought  to  be 
done  by  the  maker  or  an  experienced  mechanic. 

In  some  cases,  especially  in  microscopes  (continental 
type)  where  the  prism  is  used  in  the  fine  adjustment  the 
lubricant  in  the  prism  becomes  gummed  so  that  the 
adjustment  fails  to  respond  promptly,  and  then  jumps. 
The  bearings  should  be  thoroughly  cleaned  and  oiled 
with  paraffin  oil  or  watch  oil.  This  ought  to  be  done 
by  the  manufacturers  because  the  mechanism  is  so 
delicate  that  even  though  safely  taken  apart,  it  would 
be  put  together  and  adjusted  with  great  difficulty. 

Draw  Tube.  The  draw  tube  should  work  easily 
and  smoothly.  With  those  which  are  nickeled  and 
sliding  in  a  cloth  lined  sleeve,  little  trouble  will  be  found. 
Where  they  are  not  nickeled,  care  must  be  given  them 
similar  to  that  described  for  the  sliding  tube  coarse 
adjustment.  In  pushing  in  the  draw  tube  be  careful  not 
to  push  down  the  body  tube  and  thereby  run  the  front 
of  the  objective  into  the  cover  glass. 

Substage.  The  rules  given  above  apply  to  the  parts 
of  the  substage  in  general.  The  threads  on  the  quick 
acting  screw  on  the  instruments  so  provided  are  apt  to 
become  gummed,  making  it  hard  to  focus  the  condenser. 
This  gum  is  easily  "cut"  with  xylol  or  chloroform,  per- 
mitting the  screw  to  work  easily. 

If  the  leaves  of  the  iris  diaphragms  become  rusted  or 
gummed,  clean  them  with  xylol,  and  oil  them  thoroughly 
by  opening  and  closing  the  diaphragm  several  times  to 
evenly  distribute  the  oil  over  the  leaves.  Should  the 
leaves  become  bent  or  misplaced,  submit  them  to  the 
maker  or  a  skilled  workman. 

IMoseplece.     When   bought   at   the   same   time   the 
nosepieces  and  objectives  of  all  the  best  makers  are  now 
10 


so  made  that  the  objectives  are  parfocal:  i.  e.,  when  one 
lens  is  in  focus  the  others  on  the  nosepiece  will  be  in 
fairly  good  focus  when  they  are  swung  into  the  optical 
axis.  They  are  also  approximately  centered  so  that  a 
poinf  in  the  center  of  the  field  of  one  lens  will  be  in  the 
field  of  the  others.  To  accomplish  this,  each  set  of 
objectives  must  be  especially  fitted  to  their  particular 
nosepiece.  Care  should  then  be  taken  in  the  laboratory 
not  to  interchange  objectives.  Be  careful  not  to  bend 
the  nosepiece  in  any  way  so  that  the  objectives  will  be 
thrown  out  of  center.  Unless  you  are  positive  that  your 
lenses  are  parfocal,  always  focus  up  slightly  before  turn- 
ing from  a  lower  to  a  higher  power;  otherwise  you  are 
apt  to  swing  the  front  of  your  objective  against  the 
cover  glass  and  injure  both  the  specimen  and  objective. 
Remember  that  objectives  made  parfocal  for  one  tube 
length  arc  not  parfocal  for  a  different  length.  If  occasion 
occurs  for  screwing  the  objectives  on  or  off  from  the 
nosepiece,  always  use  both  hands,  never  letting  go  of  the 
objective  entirely,  so  as  to  preclude  any  liability  of 
injuring  them  by  dropping. 

THE   OPTICAL  PARTS 

With  the  stand  cleanliness  is  a  virtue;  with  che  ob- 
jectives it  is  also  an  absolute  necessity.  If  the  lenses 
are  dirty  they  should  be  wiped  gently  with  Japanese 
lens  paper,  which  can  be  obtained  from  any  dealer  in 
microscopical  supplies.  It  is  so  cheap  that  one  can 
hardly  afford  to  use  anything  else  on  his  lenses, — espe- 
cially the  objectives.  If  the  lens  paper  is  not  obtainable 
a  soft  old  linen  handkerchief  is  best,  providing  it  is  clean. 
Avoid  chamois  skin.  The  natural  oils  in  it  soil  the  sur- 
face of  the  lens,  and  its  aptitude  to  catch  and  hold  dirt 
makes  it  unsafe. 

Never  rub  a  lens  hard  with  anything.  Avoid  touching 
11 


the  surface  of  a  lens  with  the  bare  hands.     The  perspira- 
ion  is  hard  to  remove. 

Objectives.  If  the  front  lens  of  an  objective  be- 
comes soiled  so  that  gently  wiping  will  not  clean  it, 
breathe  upon  it  and  then  wipe  gently  with  lens  paper  or 
some  soft  linen.  If  this  does  not  remove  the  soil,  moisten 
the  paper  with  xylol  or  chloroform,  being  careful  not  to 
use  too  much.  Although  the  necessity  of  using  these 
reagents  is  unfortunate,  it  is  better  to  use  them  and  wipe 
the  lens  gently  than  to  apply  too  much  friction. 

An  immersion  objective  should  always  be  cleaned 
immediately  after  using.  It  can  then  be  cleaned  by 
gen:ly  wiping  with  a  piece  of  lens  paper.  If  the  oil  is 
allowed  to  dry,  xylol  or  chloroform  must  be  used  to 
clean  the  lens.  The  oil  collects  dust  and  grit,  which  are 
apt  to  scratch  the  lens. 

If  any  dust  settles  on  the  back  lens  of  the  objective 
it  is  best  removed  by  a  earners  hair  brush.  An  eyepiece 
should  always  be  left  in  the  tube  to  keep  dust  from 
settling  into  the  objective  on  the  lower  end. 

Never  attempt  to  take  an  objective  apart.  If  it  has  any 
ailment  serious  enough  for  this  it  is  serious  enough  to  go 
to  the  maker. 

Oculars.  The  oculars  should  be  wiped  as  directed 
for  the  objectives.  Sometimes  a  grayish  film  forms  on 
the  inner  surfaces  of  the  lenses.  This  necessitates  re- 
moving the  lenses  from  the  tube  and  wiping  their  surfaces. 

Condenser.  What  has  been  said  of  the  eyepiece 
applies  to  the  condenser.  The  inner  surfaces  of  the 
lenses  of  the  condenser  should  be  cleaned  if  the  condenser 
is  not  clear  after  cleaning  the  outer  surfaces.  The  objec- 
tive cannot  do  its  best  work  unless  the  condenser  is  clean. 

Mirror.       The  surfaces    of  the   rr.irror  demand   the 
same  care  and  treatment  as  the  lenses. 
12 


PART  II. 

THE  USE  OF  THE  MICROSCOPE 

POSITION 

In  choosing  a  place  to  work  one  should  select  a  com- 
fortable position  where  he  can  obtain  the  best  light 
available  and  have  room  for  his  microscope  and  neces- 
sary accessories  and  reagents.  There  is  some  controversy 
in  regard  to  the  using  of  the  inclination  joint.  There  is 
no  harm  in  using  it  if  it  is  more  comfortable  to  do  so. 
If  one  is  working  with  fresh  mounts  or  fluids,  the  hori- 
zontal stage  is  necessary.  Because  such  preparations  are 
so  often  used  in  the  laboratory,  it  is  best  for  one  to  train 
himself  to  use  the  microscope  with  the  tube  in  the  per- 
pendicular position  and  make  it  a  rule  to  keep  it  in  that 
position. 

Make  it  a  rule  to  work  with  both  eyes  open,  and  if 
possible,  use  either  eye  interchangeably.  A  very  little 
practice  will  enable  one  to  do  so.  By  paying  attention 
to  this  and  proper  lighting,  there  is  no  reason  why  any 
reasonable  amount  of  work  with  the  microscope  should 
injure  the  eyes. 

LIGHT 

The  best  light  is  obtained  from  white  clouds,  although 
some  authorities  claim  that  the  light  from  the  blue  sky 
is  best.  Avoid  the  use  of  direct  sunlight.  If  the  room 
is  so  situated  that  the  sun  shines  in,  use  white  shades  to 
modify  the  sunlight.  If  possible,  select  a  window  which 
is  free  from  cross  bars,  wire  nettings,  etc.,  and  which  is 
some  distance  from  swaying  branches  of  trees. 
13 


the  surface  of  a  lens  with  the  bare  hands.     The  perspira- 
ion  is  hard  to  remove. 

Objectives.  If  the  front  lens  of  an  objective  be- 
comes soiled  so  that  gently  wiping  will  not  clean  it, 
breathe  upon  it  and  then  wipe  gently  with  lens  paper  or 
some  soft  linen.  If  this  does  not  remove  the  soil,  moisten 
the  paper  with  xylol  or  chloroform,  being  careful  not  to 
use  too  much.  Although  the  necessity  of  using  these 
reagents  is  unfortunate,  it  is  better  to  use  them  and  wipe 
the  lens  gently  than  to  apply  too  much  friction. 

An  immersion  objective  should  always  be  cleaned 
immediately  after  using.  It  can  then  be  cleaned  by 
gen:ly  wiping  with  a  piece  of  lens  paper.  If  the  oil  is 
allowed  to  dry,  xylol  or  chloroform  must  be  used  to 
clean  the  lens.  The  oil  collects  dust  and  grit,  which  are 
apt  to  scratch  the  lens. 

If  any  dust  settles  on  the  back  lens  of  the  objective 
it  is  best  removed  by  a  earners  hair  brush.  An  eyepiece 
should  always  be  left  in  the  tube  to  keep  dust  from 
settling  into  the  objective  on  the  lower  end. 

Never  attempt  to  take  an  objective  apart.  If  it  has  any 
ailment  serious  enough  for  this  it  is  serious  enough  to  go 
to  the  maker. 

Oculars.  The  oculars  should  be  wiped  as  directed 
for  the  objectives.  Sometimes  a  grayish  film  forms  on 
the  inner  surfaces  of  the  lenses.  This  necessitates  re- 
moving the  lenses  from  the  tube  and  wiping  their  surfaces. 

Condenser.  What  has  been  said  of  the  eyepiece 
applies  to  the  condenser.  The  inner  surfaces  of  the 
lenses  of  the  condenser  should  be  cleaned  if  the  condenser 
is  not  clear  after  cleaning  the  outer  surfaces.  The  objec- 
tive cannot  do  its  best  work  unless  the  condenser  is  clean. 

Mirror.       The  surfaces    of  the   rrirror  demand   the 

same  care  and  treatment  as  the  lenses. 

> 

12 


PART  II. 

THE  USE  OF  THE  MICROSCOPE 

POSITION 

In  choosing  a  place  to  work  one  should  select  a  com- 
fortable position  where  he  can  obtain  the  best  light 
available  and  have  room  for  his  microscope  and  neces- 
sary accessories  and  reagents.  There  is  some  controversy 
in  regard  to  the  using  of  the  inclination  joint.  There  is 
no  harm  in  using  it  if  it  is  more  comfortable  to  do  so. 
If  one  is  working  with  fresh  mounts  or  fluids,  the  hori- 
zontal stage  is  necessary.  Because  such  preparations  are 
so  often  used  in  the  laboratory,  it  is  best  for  one  to  train 
himself  to  use  the  microscope  with  the  tube  in  the  per- 
pendicular position  and  make  it  a  rule  to  keep  it  in  that 
position. 

Make  it  a  rule  to  work  with  both  eyes  open,  and  if 
possible,  use  either  eye  interchangeably.  A  very  little 
practice  will  enable  one  to  do  so.  By  paying  attention 
to  this  and  proper  lighting,  there  is  no  reason  why  any 
reasonable  amount  of  work  with  the  microscope  should 
injure  the  eyes. 

LIGHT 

The  best  light  is  obtained  from  white  clouds,  although 
some  authorities  claim  that  the  light  from  the  blue  sky 
is  best.  Avoid  the  use  of  direct  sunlight.  If  the  room 
is  so  situated  that  the  sun  shines  in,  use  white  shades  to 
modify  the  sunlight.  If  possible,  select  a  window  which 
is  free  from  cross  bars,  wire  nettings,  etc.,  and  which  is 
some  distance  from  swaying  branches  of  trees. 
13 


For  long  continued  work  on  any  one  subject  artificial 
light  has  one  advantage  over  daylight  in  that  it  is  con- 
stant in  quality  and  intensity.  The  best  artificial  light 
is  a  Welsbach  burner.  A  whitened  incandescent  bulb 
is  good.  Ordinary  lamplight  can  be  used  very  success- 
fully. In  using  artificial  light  it  is  best  to  use  a  bull's  eye 
condenser  between  it  and  the  mirror.  It  is  also  best, 
wherever  possible,  to  use  a  blue  glass  between  the  light 
source  and  the  specimen.  Some  workers  make  a  glass 
globe  filled  with  ammonia  copper  sulphate  serve  the 
purpose  of  both  the  condenser  and  the  blue  glass.  It  is 
so  mounted  in  a  shade  as  to  exclude  all  other  light  from 
the  microscope.  An  eye  shade,  or  some  shade  cutting 
off  all  light  from  the  microscope  excepting  that  which 
strikes  the  mirror,  is  often  desirable. 

FOCUSING 

After  seeing  that  an  objective  (low  power)  and  an  ocu- 
lar are  in  place  put  a  transparent  or  semi-transparent 
specimen  on  the  stage,  swing  the  mirror  bar  to  the  me- 
dian line,  take  hold  of  the  edge  of  the  mirror  and  adjust  it 
so  as  to  illuminate  the  object  as  evenly  as  may  be  judged 
by  looking  directly  at  it. 

Focus  the  body  tube  down  by  means  of  the  coarse 
adjustment  until  the  objective  nearly  touches  the  cover 
glass,  being  careful  not  to  touch  it.  Then  with  the  eye 
at  the  eyepiece,  focus  up  carefully  with  the  coarse  ad- 
justment until  the  specimen  comes  plainly  into  view. 
Be  careful  not  to  pass  by  this  focal  point  without  notic- 
ing it.  This  is  likely  to  occur  if  the  light  be  too  intense 
and  the  specimen  thin  and  transparent.  If  the  sliding 
tube  coarse  adjustment  is  used,  focus  carefully  by  giving 
the  tube  a  spiral  movement. 

When  the  object  is  brought  fairly  well  into  focus  by 
means  of  the  coarse  adjustment  use  the  fine  adjustment 
14 


to  obtain  the  sharpest  focus  to  bring  out  details.  Do 
not  expect  too  great  a  range  in  the  fine  adjustment.  It 
is  even  more  dangerous  to  focus  down  to  any  extent  with 
the  fine  adjustment  than  with  the  coarse  adjustment, 
because  any  impact  of  the  front  of  the  objective  on  the 
cover  cannot  be  as  easily  felt.  \Vhile  moving  the  speci- 
men about  to  observe  different  parts  of  it,  it  will  be 
necessary  to  continually  work  the  fine  adjustment  to 
keep  the  object  in  focus.  It  is  always  well  to  move  the 
specimen  when  trying  to  get  a  focus,  for  without  the 
movement  one  may  be  trying  to  focus  upon  a  point 
where  there  is  no  object,  and  again,  the  moving  object 
is  more  apt  to  be  noticed  as  the  lens  comes  into  focus. 

It  will  be  noticed  during  this  movement  that  the 
microscope  reverses  the  image,  and  that  the  specimen 
seems  to  move  in  the  direction  opposite  to  that  in  which 
it  is  moved.  This,  along  with  the  fact  that  the  micro- 
scope magnifies  the  movement  as  well  as  the  specimen, 
is  perplexing  at  first  and  makes  it  difficult  to  move  the 
specimen  just  where  it  is  wanted,  and  no  farther.  With 
practice  comes  the  delicacy  of  movement  which  enables 
one  to  put  the  specimen  just  where  he  wants  it. 

The  beginner  should  always  use  the  low  power  objec- 
tives and  oculars  first.  The  low  power  objectives  have 
longer  working  distances  and  are  not  so  apt  to  be  in- 
jured. They  always  show  a  larger  portion  of  the  speci- 
men and  thus  give  one  a  better  idea  of  the  general 
contour.  After  obtaining  this  general  idea  the  higher 
powers  can  be  used  to  bring  out  greater  detail  in  any 
particular  part.  If  the  objectives  are  par-focalized  and 
centered  on  a  nosepiece  as  described  on  page  II,  the 
change  of  objectives  is  made  by  simply  turning  one 
objective  out  of  the  optkal  axis  and  the  other  into  it 
without  the  necessity  of  re-focusing  (except  for  a  slight 
turn  of  the  fine  adjustment)  and  again  hunting  up  the 
15 


particular  spot  desired,  for  if  this  spot  is  in  the  center  of 
the  field  of  the  low  power  it  will  be  somewhere  in  the 
field  of  the  higher  power.  It  is  too  much  to  ask  of  the 
maker  that  the  lenses  be  made  absolutely  parfocal  and 
centered.  The  delicacy  of  the  centering  can  be  appre- 
ciated when  the  magnification  and  the  extremely  small 
portion  examined  is  considered.  When  the  objectives 
are  not  thus  fitted  to  the  nosepiece,  re-focusing  and 
again  hunting  up  the  object  are  necessary.  In  doing  so 
we  repeat  the  caution  to  always  focus  up  before  turning 
the  nosepiece.  When  no  revolving  nosepiece  is  used 
the  change  of  objectives  .means  the  unscrewing  of  one 
and  the  screwing  of  the  other  into  its  place,  and  re- 
focusing  as  before. 

ILLUMINATION— WITHOUT  SUBSTAGE   CONDENSER. 

Central  Light.  It  has  been  necessary  in  the  fore- 
going paragraphs  to  secure  some  light  upon  the  specimen, 
but  no  directions  have  been  given  as  to  the  proper 
illumination  of  the  same.  Accuracy  of  results  depends 
upon  correct  illumination  more  than  any  other  one 
thing.  A  vast  majority  of  all  microscopic  work  is  done 
by  light  transmitted  through  transparent  or  semi-trans- 
parent objects.  We  will  at  present  consider  only  such 
objects.  The  matter  of  illuminating  opaque  objects  will 
be  taken  up  later.  The  mirror  is  placed  below  the  stage 
as  a  convenient  means  of  reflecting  the  light  through 
the  object  into  the  objective.  It  is  plane  on  one  side  and 
concave  on  the  other.  The  concave  mirror  is  always 
used  when  the  substage  condenser  is  not  used,  except  in 
the  case  of  very  low  power  objectives,  when  it  is  best  to 
use  the  plane  mirror. 

When  the  light  is  thrown  upon  the  specimen  and  the 
objective  focused  as  previously  directed,  remove  the 
ocular  and  look  into  the  tube  at  the  back  lens  of  the 
16 


objective.  With  the  medium  and  higher  power  (i6mm 
and  above)  objectives  the  minimized  image  of  the  mirror 
with  its  mounting  will  be  seen.  Swing  the  mirror  bar  to 
the  median  line  and  as  nearly  as  possible  arrange  the  mir- 
ror so  that  its  mounting  will  be  concentric  with  the  per- 
iphery of  the  back  lens  of  the  objective.  All  of  the  better 
microscopes  are  made  with  a  ''center  stop,"  indicating 
when  the  mirror  bar  is  in  a  line  parallel  with  the  optical 
axis  of  the  microscope.  This  is  done  because  central,  or 
axial,  light  gives  a  symmetrical  illumination,  which  is 
best  for  observing  the  large  proportion  of  transparent 
objects.  This  in  itself  does  not  insure  axial  light.  The 
mirror  must  be  so  turned  that  the  rays  of  light,  or  the 
axis  of  the  cone  of  light,  reflected  from  it  enter  the  objec- 
tive parallel  to  its  axis..  This  cannot  always  be  done. 
Other  considerations  are  more  important  than  exact 
central  light.  In  working  with  daylight,  reflections  from 
trees,  window  sash,  etc.,  are  apt  to  be  seen  on  the  mirror. 
If  the  whole  microscope  cannot  be  so  shifted  as  to  clear 
the  mirror  of  these  reflections  the  mirror  itself  should  be 
turned  so  that,  if  possible,  no  images  will  appear  upon  it. 

If  artificial  light  is  used  the  mirror  should  be  so  turned 
that  the  image  of  the  light  is  seen  in  the  center  of  it. 
The  more  nearly  this  image  covers  the  mirror,  the  better. 
If  a  bulKs  eye  condenser  is  at  hand,  so  place  it  between 
the  light  source  and  the  object  that  a  sharp  image  of 
the  light  source  will  be  seen  in  the  center  of  the  back 
lens  of  the  objective. 

If  the  above  rules  are  followed  it  will  be  found  upon 
replacing  the  eyepiece  that  the  field  is  evenly  illumi- 
nated. It  may  be  necessary  to  vary  the  width  of  the  cone 
and  the  quantity  of  light  by  use  of  the  diaphragm  which 
is  always  placed  on  all  the  better  microscopes  as  nearly 
as  possible  even  with  trie  top  surface  of  the  stage. 
"When  no  condenser  is  used  the  size  of  the  opening  in 
17 


the  diaphragm  should  be  about  that  of  the  front  lens 
of  the  objective.  For  some  objects  and  some  objectives 
this  rule  may  be  quite  widely  departed  from;  one  must 
learn  by  trial."*  The  concave  mirror  acts  as  a  lens  and 
has  a  focus  like  a  lens.  It  will  often  be  found  that  by 
carefully  focusing  the  mirror,  details  will  be  brought 
out  clearly  which  otherwise  would  be  but  dimly  seen. 

It  may  be  found  that  in  focusing  up  and  down,  the 
image  shifts  slightly  from  right  to  left,  or  to  and  fro. 
This  may  possibly  be  due  to  an  imperfection  in  the 
microscope,  but  if  the  instrument  is  in  good  repair,  and 
from  any  one  of  the  reputable  makers,  the  chances  are 
more  than  likely  that  the  shifting  is  due  to  oblique  light, 
even  though  the  mirror  bar  may  be  in  the  median  line. 
This  is  even  more  apparent  with  a  condenser  than  with- 
out it.  Manufacturers  are  often  condemned  because  of 
a  mistaken  idea  that  the  mirror  bar  in  this  position 
means  axial  light.  A  slight  turn  of  the  mirror  will  stop 
the  shifting  and  give  axial  illumination.  When  there  is 
no  lateral  motion  in  focusing,  the  light  is  centered. 

Oblique  Light.  Some  objects,  such  as  diatoms, 
rulings,  etc.,  are  better  defined  when  oblique  light  is 
used.  This  is  accomplished  without  the  condenser  by 
swinging  the  mirror  out  of  the  optical  axis  and  so  turning 
it  as  to  throw  as  much  light  as  possible  upon  the  object. 
When  the  ocular  is  removed  the  image  of  the  mirror  will 
be  seen  at  one  side  o  the  center  of  the  back  lens  of  the 
objective.  When  focusing  a  decided  lateral  motion  of 
the  object  will  be  noticed. 

ILLUMINATION— WITH  SUBSTAGE    CONDENSER 
Central  Light.     All  of  the    better  microscopes  are 
provided  with  a  condenser  fitted  beneath  the  stage,  which 
brings  parallel  rays  of  light  to  a  focus  at  a  point  above 
*The  Microscope. — Gage. 

18 


its  upper  surface.  With  the  lowest  powers  a  condenser 
is  not  needed,  but  for  the  medium  and  higher  powers  the 
condenser  not  only  furnishes  the  amount  of  light  needed, 
but  provides  an  easy  means  of  providing  each  objective 
with  a  cone  of  light  suitable  to  its  aperture. 

Condensers  are  of  two  great  classes — the  archomatic 
and  non-achromatic.  The  achromatic  are  much  the 
better,  and  are  indispensable  for  photo-micrographic 
work,  but  as  the  non-achromatic  (Abbe)  is  in  such  general 
use  on  account  of  its  lesser  cost  and  because  it  is  suffi- 
cient for  ordinary  work,  we  will  consider  it  especially. 
The  same  general  rules  apply  to  both,  which  are  funda- 
mentally the  same  as  those  given  before,  with  the  excep- 
tion that  in  most  cases  the  plane  mirror  is  used  because, 
as  stated  above,  the  condenser  is  made  for  parallel  rays 
of  light. 

After  removing  the  ocular,  turn  the  mirror  so  that  the 
back  lens  of  the  objective  is  fully  and  evenly  illuminated 
and,  if  possible,  free  from  any  images  of  trees,  window 
sash,  etc.  If  these  images  cannot  be  dispelled  by  turning 
the  mirror,  use  the  concave  mirror.  Slightly  lowering 
the  condenser  will  also  accomplish  the  end.  There  is  an 
objection  to  both  of  these  methods,  which  will  be  ex- 
plained later. 

When  this  is  accomplished  the  proper  cone  of  light 
must  be  secured  by  opening  or  closing  the  diaphragm 
below  the  condenser. 

A  good  general  rule  is  to  close  the  diaphragm  so  that 
in  looking  at  the  back  lens  of  the  objective  the  diaphragm 
opening,  which  can  be  plainly  seen,  appears  to  be  about 
half  the  diameter  of  the  back  lens  of  the  objective  when 
it  is  in  focus.  Then  with  the  ocular  in  place  change  the 
opening  to  give  the  best  results.  The  thinner  the  tissues 
and  the  greater  the  contrasts  the  larger  the  cone  of  light 
which  may  be  used.  Thicker  tissues  and  those  with  less 
19 


contrast  require  a  narrower  cone,  gaining  thereby  greater 
depth  of  sharpness  (penetration).  The  narrower  the 
cone  the  flatter  the  field  appears.  Very  few  objects 
permit  of  a  cone  which  fills  the  back  lens  of  the  objective 
and  in  no  case  should  the  diameter  of  the  iris  diaphragm 
appear  to  be  larger  than  the  diameter  of  the  back  lens 
when  the  ocular  is  removed. 

When  objectives  of  over  i.o  N.  A.  (see  p.  25)  (immer- 
sion objectives)  are  used,  the  full  aperture  of  the  con- 
denser cannot  be  utilized  without  immersing  it,  i.  e., 
placing  a  drop  of  oil  between  it  and  the  lower  surface 
of  the  slide.  This  is  seldom  practiced  in  general  labora- 
tory work  on  account  of  its  inconvenience,  but  it  is 
necessary  to  the  most  critical  work. 

In  working  with  artificial  light  it  is  always  best  to  use 
a  bull's  eye  condenser.     If  no  bull's  eye  is  available,  use 
the  concave  mirror  and  so  turn  it  that  the  image  of  the 
light  source  appears  in  the  centre  of  the  back  lens  of  the 
objective  when  the  ocular  is  removed.     When  the  bull's 
eye  is  used,  select  the  plane  mirror  and  so  place  the  light 
source  and  bull's  eye  that  the  image  of  the  light  source 
appears  natural  size  on  a  cardboard  placed  at  the  back 
of  the  condenser.     Remove  the  bull's  eye  and  put  the 
light  source  in  its  place.     Focus  the  substage  condenser 
so  that  the  image  of  the  light  source  appears  in  the  plane 
of  the  object.     This  is  best  seen  by  using  a  low  power 
objective   and   ocular.     Now  put   the   bull's  eye  in  its 
former  position  after  removing  the  light  to  its  original 
place,  or  as   before,  so   arrange  the  light  source  that  a 
sharp  image  of  the    light  source   appears  on   the  back 
of  the  condenser  iris,  or  a  card  placed  against  it.     This 
will  give  an  even  illumination.    A  blue  glass  should   also 
be   used   beneath   the  condenser,  unless  the  blue  globe 
mentioned  on  page  14  is  used.     This  modifies  theyello\v 
artificial  light. 

>  20 


Oblique  Light.  Even  though  both  the  condenser 
and  iris  are' centered,  central  or  axial  light  is  not  ob- 
tained, unless  the  rays  of  light,  or  the  axis  of  the  cone 
of  light,  from  the  mirror  enter  the  condenser  parallel 
with  its  axis.  This  fact  is  often  forgotten  as  was  pointed 
out  before. 

Beside  the  turning  of  the  mirror  oblique  light  can  be 
obtained  in  a  greater  degree  by  decentering  the  lower 
iris  and  in  the  best  microscopes  the  obliquity  can  be 
obtained  from  any  azimuth'by  revolving  the  diaphragm 
mounting. 

With  the  simple  diaphragm  mounting  with  a  ring 
beneath  the  diaphragm  for  blue  glass  oblique  light  can 
be  obtained  by  slipping  a  card  between  the  ring  and 
diaphragm  in  such  a  manner  as  to  let  the  light  into  one 
side  of  the  condenser  only. 

ILLUMINA  TION—OPA  QUE 

There  are  some  objects  which  cannot  be  made  trans- 
parent and  must  be  examined  by  reflected  light.  When 
low  powers  are  used  and  the  mirror  brought  above  the 
stage  the  concave  mirror  is  sometimes  sufficient.  The 
bull's  eye  condenser  gives  better  results.  In  using  it 
some  care  must  be  exercised  to  light  the  object  and  at 
the  same  time  cast  as  few  shadows  as  possible. 

Where  high  powers  with  consequent  short  working 
distances  are  necessary,  light  must  be  thrown  down 
through  the  objective  and  reflected  back  again  to  the 
eye  by  a  prism  placed  in  the  objective  mount  or  above 
it.  Artificial  light  is  necessary  with  such  an  arrangement. 

FOCUSING  THE  CONDENSER 

Nearly  every  substage  is  provided  with  a  means  for 

focusing    the  condenser.     The  condenser  does   its   best 

work  only  when   the  rays   of  light   passing   through   it 

are    focused    sharply  on   the   object.     If  there  are  any 

21 


reflections  from  the  mirror  of  trees,  window  sash,  etc  , 
they  will  show  on  the  back  lens  of  the  objective  when 
it  is  focused  and  when  the  condenser  is  in  this  posi- 
tion. The  operator  must  decide  for  himself  as  to 
whether  he  gets  better  results  with  the  sharp  focus  and 
the  images,  or  without  either.  If  he  decides  to  retain 
the  images  he  can  get  best  results  by  turning  the  mir- 
ror so  that  they  are  as  symmetrically  distributed  over 
the  back  lens  as  possible.  This  emphasizes  the  impor- 
tance of  a  clear,  open  source  of  light. 

CENTERING  THE  CONDENSER 

For  central  light  the  axis  of  the  condenser  should 
coincide  with  the  axis  of  the  objective  and  the  center 
of  the  opening  of  the  diaphragm  beneath  the  conden- 
ser should  also  be  in  this  axis.  On  most  of  the  medium 
grade  microscopes  the  iris  is  so  fastened  to  the  conden- 
ser that  it  is  concentric  with  its  axis,  and  both  are  gen- 
erally centered  with  the  optical  axis  of  the  body  tube 
before  leaving  the  factory.  This  can  be  tested  by  a 
simple  means.  Close  the  diaphragm  to  its  smallest  ap- 
erture and  notice  this  aperture  through  the  back  lens 
of  the  objective.  If  the  condenser  is  centered  this 
opening  will  appear  in  the  center  of  the  lens  and  will 
remain  concentric  with  the  periphery  of  the  lens  when 
the  objective  is  focused  up  and  down.  The  periphery 
of  the  top  surface  of  the  condenser  will  also  be  con- 
centric with  the  periphery  of  the  back  lense  of  the  ob- 
jective. 

On  the  more  expensive  microscopes  there  are  little 
screws  provided  for  moving  the  condenser  in  and  out  of 
center.  By  means  of  these  the  condenser  can  be  cen- 
tered by  observing  the  above  rule.  On  these  instru- 
ments the  iris  diaphragm  is  brought  in  and  out  of  center 
by  a  rack  and  pinion,  a  click  indicating  when  it  is  cen- 
tered. 

22 


TUBE  LENGTH  AND  COVER   CLASS 

All  objectives  are  corrected  to  a  certain  tube  length 
(i6omm  by  most  makers — Leitz  i/omm)  and  all  objec- 
tives in  fixed  mounts  of  over  .70  N.  A.  are  corrected  to  a 
definite  thickness  of  cover  glass  as  well.  (Zeiss  .I5mm 
.2Omm;  Leitz,  .I7mm;  Bausch  &  Lomb  and  Spencer, 
.i8mm).  These  objectives  give  their  best  results  only 
when  used  with  the  cover  glass  and  tube  length  for  which 
they  are  corrected.  As  indicated  in  the  frontispiece, 
the  tube  length  extends  from  the  eye  lens  of  the  eyepiece 
to  the  end  of  the  tube  into  which  the  objective  or  nose 
piece  is  screwed.  If  a  nosepiece  is  used  the  draw  tube 
must  be  correspondingly  shortened.  If  the  cover  glass 
is  thinner  than  that  for  which  the  objective  is  corrected 
the  tube  must  be  lengthened  to  obtain  best  results; 
T  thicker,  shortened.  The  more  expensive  objectives 
are  provided  with  adjustable  mounts  by  which  the  dis- 
tances between  the  lens  systems  may  be  changed  to  com- 
pensate for  difference  of  thickness  of  cover.  They  are 
successfully  used  only  in  the  hands  of  an  expert.  One 
of  them  out  of  adjustment  is  worse  than  an  ordinary 
objective. 

EQUIPMENT 

A  microscope  equipment  may  be  very  elaborate  and 
some  work  requires  such  an  outfit,  but  every  microscope 
to  be  efficient  should  be  provided  with  at  least  two 
objectives,  and  preferably  two  oculars.  If  but  one  ocular 
is  bought  the  6x  (No.  2,  or  i  J")  is  best.  If  two  are  pro- 
vided, the  4x  and  8x  (No.  I  and  3,  or  2"  and  i")  are  pref- 
erable. For  ordinary  biological,  histological  and  patho- 
logical work  the  most  desirable  and  most  universally 
used  objectives  are  the  i6mm  (§")  and  4mm  (£").  The 
equivalents  of  these  in  European  nomenclature  are  Nos. 
3  and  6,  or  A  and  D,  according  to  the  maker.  In  some 
cases  where  a  higher  power  is  desired  a  3mm  (-J-"  or  No.  7) 


is  used.  For  counting  blood  corpuscles  a  5mm  (3-")  or 
a  narrow  angled  4mm  (•£")  with  a  long  working  distance 
for  working  through  the  thick  cover  of  the  blood  counter 
is  used.  For  entomological  work  and  other  low  power 
work  a  4Omm  (if")  is  very  desirable.  If  bacteriological 
work  or  special  cytological  work  is  to  be  done,  an  immer- 
sion objective  is  indispensable.  The  2mm  (yVO  1S  most 
used.  For  extreme  work  the  i.5mm  (yVO  ls  called  into 
use.  An  immersion  objective  is  one  which  is  so  corrected 
that  a  drop  of  oil  (water,  if  it  is  a  water  immersion) 
must  be  used  between  the  front  lens  and  the  cover  glass. 
They  are  used  like  any  other  objective  excepting  for  the 
placing  of  this  drop  of  oil  and  for  the  fact  that  their 
shorter  working  distance  requires  greater  care  in  hand- 
ling. It  is  best  to  place  the  drop  of  oil  upon  the  front 
lens  of  the  objective  with  the  little  wire  or  rod  which 
accompanies  every  immersion  oil  bottle.  Any  bubbles 
or  dirt  in  the  oil  can  be  more  easily  detected  in  this  way. 
It  is  a  little  unhandy  to  do  this  where  the  objective  is 
screwed  on  the  nosepiece.  Many  workers  prefer  to  put 
the  drop  on  the  cover  below  the  objective.  Great  care 
should  be  taken  to  exclude  dirt  or  air  bubbles.  Dirt 
particles  are  apt  to  scratch  the  lens,  and  bubbles  set  up 
refractions  which  greatly  interfere  with  the  well  working 
of  the  lens.  If  the  bubbles  are  present  they  can  be  easily 
seen  by  removing  the  ocular  and  looking  down  into  the 
objective.  They  must  be  removed  even  though  it  be 
necessary  to  wipe  off  all  the  oil  and  start  over.  The 
bubbles  are  very  apt  to  occur  if  the  oil  contact  is  broken 
several  times  in  an  attempt  to  focus.  If  the  full  aper- 
ture of  the  objective  (or  any  aperture  greater  than  N.  A. 
i.o)  is  to  be  used  the  condenser  must  also  be  immersed 
as  described  on  page  20. 

Where  the  very  best  possible  resu.ts  are  demanded  the 
apochromatic  objectives  with  compensating  oculars  are 
24 


used.  They  are  made  in  the  same  powers  as  indicated 
above  and  are  superior  to  the  achromatic  lenses  because 
they  have  a  more  perfect  chromatic  and  spherical  cor- 
rection. 

OPTICAL  QUALITIES   OF  OBJECTIVES 

Numerical  Aperture.  N.  A.=n.  sin  u.  This 
term  was  introduced  by  Abbe.  n.  is  the  refractive  index 
of  the  medium  between  the  object  and  the  front  lens  of 
the  objective  (air  in  case  of  dry  objectives  and  water  or 
oil  in  case  of  immerson  objectives),  and  u  is  half  the 
angular  aperture. 

Several  important  qualities  of  the  objective  depend 
upon  the  numerical  aperture. 

(a)  RESOLVING  POWER.     This  is  directly  proportion- 
al to  the  numerical  aperture  and  represents  the  ability 
of  the  objective  to  show  detail  in  the  image  of  the  object. 
The  higher  the  numerical  aperture,  the  greater  the  re- 
solving power,  and  the  finer  the  detail  we  may  expect  to 
see  in  the  image. 

(b)  DEPTH  OF  SHARPNESS,  OR  PENETRATION.    This 
is  the  power  of  an  objective  to  show  sharply  objects 
lying  in  different  planes,  one  above  another,  without 
the  necessity  of  focusing  up  and  down.     The   depth   of 
sharpness  is  in  inverse  ratio  to  the  numerical  aperture. 
Therefore  the  lens  of  low  numerical  aperture  has  little 
resolving  power  and  great  penetration.    The  lens  of  high 
numerical  aperture  has  great  resolving  power  and  little 
penetration, — unless  it  be  used  with  a  narrow  cone  of 
light  which  practically  makes  it  a  lens  of  low  aperture 
with  the  qualities  of  such  a  lens. 

(c)  ILLUMINATING    POWER.     The    brilliancy   of  the 
objective  increases  with   the   square  of  the  numerical 
aperture  of  the  objective.     An  objective  of  .40  N.  A. 

25 


will  give  an  image  four  times  as  brilliant  as  one  of  .20 
N.  A.,  provided  the  magnification  is  the  same  and  the 
full  cone  of  the  illumination  is  used  in  both  cases. 

Magnifying  Power.  The  magnifying  power  of  an 
objective  is  in  inverse  ratio  to  its  focal  distance.  An 
objective  of  2mm  focal  distance  will  give,  with  the  same 
ocular,  a  magnification  eight  times  greater  than  one  of 
1 6mm  focal  distance.  Numerical  aperture  and  mag- 
nifying power  are  of  little  advantage  if  the  definition  is 
not  good. 

Definition.  The  definition  of  an  objective  is  char- 
acterized by  the  cleanness  and  sharpness  of  the  outlines 
of  the  image. 

Definition  depends  upon  the  corrections  for  chromatic 
and  spherical  aberrations,  and  the  workmanship; — the 
centering  of  the  lenses,  etc. 

CHROMATIC  ABERRATION  is  due  to  the  fact  that  a  ray 
of  white  light  passing  from  one  medium  to  another  of 
different  refractive  index  at  any  angle  other  than  90° 
to  the  surface  between  them  is  refracted  and  dispersed 
into  its  component  colors. 

SPHERICAL  ABERRATION  is  due  to  the  fact  that  a 
spherical  surface  cannot  bring  a  beam  of  light  which 
passes  through  its  vertex  to  the  same  focus  as  that  of 
a  beam  of  light  passing  through  any  other  zone. 

Both  aberrations  are  corrected  by  the  use  of  different 
kinds  of  glass  (crown  and  flint)  combined  as  double  and 
triple  lenses  in  the  objective.  Neither  can  be  corrected 
absolutely  for  all  colors  in  an  achromatic  objective. 
Apochromatic  objectives  approach  the  ideally  corrected 
objective  almost  to  perfection. 

An  objective  can  be  tested  for  chromatic  correction 
by    using    a    narrow  cone  of  oblique  light  and  a  coarse 
26 


grating.  Abbe's  test  plate  is  best.  Diatoms  are  good. 
No  stained  object  should  be  used. 

If  the  spherical  correction  is  perfect  (see  next  para- 
graph) and  one  side  of  a  line  passing  through  the  center 
of  the  field  shows  a  clear,  narrow,  greenish  yellow  border, 
while  the  other  side  is  fringed  with  a  violet  red  (second- 
dary  colors)  the  objective  is  chromatically  corrected. 
The  colors  shown  in  the  higher  power  objectives  are  of  a 
more  primary  character,  /.  e.,  nearer  the  yellow  and  blue. 
Apochromatic  objectives  showno  color  borders  in  this  test. 

The  spherical  correction  of  an  objective  is  perfected 
for  a  certain  thickness  of  cover  glass  and  a  certain  tube 
length,  and  is  influenced  greatly  by  any  variation  in 
either.  This  is  especially  true  with  the  high  power  dry 
objectives.  The  homogeneous  immersion  objectives  are 
not  sensitive  to  the  variation  in  the  cover  thickness 
because  the  immersion  oil  between  the  cover  glass  and 
the  lens  is  of  the  same  refractive  index  as  the  glass. 
They  must  be  used  however,  with  the  proper  tube 
length.  In  testing  an  objective  for  its  spherical  correc- 
tion it  is  therefore  very  important  to  supply  the  proper 
thickness  of  cover  and  tube  length.  It  is  manifestly 
unfair  to  judge  an  objective  on  this  point  without  com- 
plying with  these  conditions.  The  test  for  spherical 
correction  can  be  made  on  the  same  object  as  used  for 
the  chromatic  test.  If  the  edges  of  the  lines  in  the  center 
of  the  field  appear  equally  sharp  and  clear  when  illum- 
inated by  either  a  narrow  central  cone  of  light  or  a 
narrow  oblique  cone  without  having  to  change  the  fine 
adjustment  the  objective  is  spherically  corrected.  The 
color  remnants  mentioned  above  will  be  clear  and  trans- 
parent, while,  if  the  lens  is  poorly  corrected  spherically, 
these  borders  will  appear  muddy  and  turbid.  Defects 
in  spherical  corrections  can  often  be  corrected  by  using 
cover  glasses  suitable  to  them,  also  by  changing  the  tube 
27 


length.  The  fact  that  the  periphery  of  the  field  is  not 
in  focus  at  the  same  time  as  the  center  does  not  bespeak 
a  lack  of  spherical  correction,  but  a  lack  of  flatness  of 
•field  with  which  it  is  often  confounded. 

Flatness  of  Field  depends  not  only  upon  the  ob- 
jective itself,  but  upon  the  ocular  and  the  cone  of  light 
used,  whereas  the  spherical  aberration  is  inherent  in  the 
objective  itself.  No  field  is  absolutely  flat.  It  is  a 
desirable  quality  in  a  lens  but  spherical  and  chromatic 
corrections  should  never  be  sacrificed  for  it.  Some 
lenses  appear  to  be  "flatter"  than  they  really  are,  because 
their  corrections  are  so  poor  that  little  contrast  is 
noticed  between  objects  in  the  center  of  the  field  and  at 
the  edge.  Narrow  cones  of  light  give  a  flatter  field  than 
wide  ones.  Thin  objects  are  more  critical  tests  for 
flatness  of  field  than  thick  ones. 

Working  Distance  is  the  free  distance  between  the 
cover  glass  and  the  objective  when  the  latter  is  foccrsed. 
It  decreases  generally  with  increasing  power  and  numeri- 
cal aperture  of  the  objective.  Of  two  lenses  with  the 
same  focal  distance  the  one  with  the  higher  N.  A.  will 
have  the  shorter  working  distance.  The  working  dis- 
tance also  depends  on  the  mounting  of  the  front  lens. 
If  the  lens  has  a  prominent  mounting  projecting  beyond 
its  surface  the  working  distance  is  lessened  thereby. 
THE  OCULARS 

A  certain  magnification  by  the  ocular  will  be  neces- 
sary, and  sufficient,  to  bring  out  all  the  detail  in  the 
image  which  can  be  secured  from  the  numerical  aper- 
ture of  the  objective.  If  we  use  a  higher  ocular  we  lose 
depth  of  sharpness  and  size  of  field,  since  they  are  both 
inversely  proportional  to  the  magnification.  We  also 
lose  illumination,  which  varies  inversely  as  the  square  of 
the  magnification. 


We  therefore  get  the  greatest  effectiveness  out  of  an 
objective, — the  largest  field,  the  greatest  penetration,  and 
the  best  illumination, — by  using  the  lowest  magnifica- 
tion which  makes  all  the  detail  in  the  image  visible.  If 
we  increase  the  magnification  beyond  this  point  we  do 
so  at  the  expense  of  other  good  qualities. 

Lengthening  the  tube  increases  the  magnification 
proportionately. 

FINAL   HINTS 

Sometimes  the  worker  may  have  faithfully  carried  out 
all  the  directions  heretofore  given  and  been  assured 
that  his  lenses  possess  the  above  named  qualities  as  they 
ought,  yet  be  unable  to  obtain  the  desired  results.  He 
may  be  working  with  a  water  mount  and  his  dry  objec- 
tive become  "immersed"  in  some  water  which  has 
worked  to  the  top  of  the  cover  glass.  His  objective  may 
be  dirty  from  a  previous  "immersion,"  or  it  may  have 
some  other  dirt  upon  the  front  lens.  The  field  may  be 
covered  with  specks  which  revolve  when  the  ocular 
is  turned.  The  field  may  be  dim  or  hazy,  due  to  dirt 
on  the  back  of  the  objective  or  a  film  on  the  inner  sur- 
faces of  the  lenses  of  the  ocular,  or  because  of  moisture 
settling  on  the  lenses  because  they  have  just  been  brought 
from  a  cold  into  a  warm  room.  He  may  see  great  streaks 
on  his  field,  which  are  due  to  his  own  eye  lashes,  or  he 
may  see  small,  slowly  moving  bodies  floating  across  the 
field.  With  the  exception  of  this  last,  the  ailment  has 
only  to  be  mentioned  to  suggest  the  remedy.  The 
muscae  volitantes,  as  these  last  named  bodies  are  called, 
are  little  specks  or  shreds  in  the  vitreous  humor  of  the 
eye  which  cannot  be  removed,  but  which  can  easily  be 
disregarded. 

In  water  mounts  and  fresh  balsam  mounts  one  is  apt 
to  find  air  bubbles.  Tc  be  sure, that  the  ^object  is  an 
29  -. 


air  bubble,  focus  up  with  central  light.  The  bright 
spot  in  the  center  will  become  clearer  while  the  edge  will 
become  darker.  With  oblique  light  the  bright  spot  will 
be  thrown  to  one  side.  In  studying  water,  blood  or  any 
fluid,  always  cover  the  drop  with  a  cover  glass.  The 
objectives  are  corrected  for  rays  passing  through  media 
with  parallel  surfaces.  If  such  a  mount  is  not  kept 
horizontal,  currents  will  be  set  up,  due  to  gravitation, 
and  they  will  be  seen  with  a  magnified  velocity  seemingly 
running  up  hill. 

The  fact  that  the  microscope  reverses  every  move- 
ment and  magnifies  it  may  be  mentioned  again. 

Beside  any  movement  due  to  currents  there  is  some- 
times a  peculiar  indefinite  to  and  fro  movement  of  par- 
ticles from  one  position  to  another.  This  is  called  Brown- 
ian  movement. 

In  studying  sections  a  true  idea  of  the  structure  of 
the  tissue  can  only  be  obtained  by  moving  the  slide 
about  to  bring  different  parts  into  the  optical  axis  and 
by  focusing  with  the  fine  adjustment  to  bring  different 
levels,  or  optical  planes,  successively  into  view.  Where 
serial  sections  are  used  each  section  must  be  studied 
in  relation  to  its  neighbors. 

Sometimes  sections  which  are  freshly  mounted  in 
balsam  appear  cloudy  and  indistinct.  This  is  because 
of  failure  to  thoroughly  dehydrate  the  specimen  before 
putting  it  into  the  balsam.  But  this  brings  us  into  the 
realm  of  laboratory  technique  which  is  beyond  the  scope 
of  this  little  volume, 


This  is  Our 

Latest 

Microscope 


No.  20 


IN  THIS  Microscope  we  have 
embodied  all  the  desirable 
features  ot  the  old  construc- 
tion and  replaced  other  features 
with  improvements   never   be- 
fore applied  in  this  way. 
The  solid  bronze  arm  provides  a  convenient  handle  for 
carrying  the  microscope,  and  our  new   cone  movement, 
anti-friction  bearing,  fine  adjustment  furnishes  a  smooth 
steady  movement  which  is  unsurpassed. 

The  vulcanite-covered  stage  is  extra  large,  giving  ample 
room  for  all  kinds  of  work.  The  whole  is  beautifully  fin- 
ished. It  is  the  handiest,  handsomest  microscope  on  the 
market. 

20D. — With  2    oculars,     double     nosepiece. 

16mm  and  4mm  objectives  with  upper   iris 

diaphragm  ....... 

20F. — Same  as  above  with  condenser  and  iris 

•  diaphragm 

20H. — With  2  oculars,  triple  nosepiece,  16mm, 

4mm   and  2mm   oil  immersion   objectives, 

condenser  and  iris  diaphragms 


NET 


$  57.00 


$  64.50 


$  90.00 


Spencer  Lens  Co.,  Buffalo,  N.  Y. 


SPENCER 
LENS  CO/S 

No.  45 
Microscope 


AS  A  PRACTICAL  working 
Microscope  at  a  reason- 
able  price    this  instru- 
ment is  unsurpassed.     It  com- 
and    convenience. 


No.  45 

bines    steadiness,    durability,   beauty 

The  long  range,  fine  adjustment  is  absolutely  dust  proof. 
The  fact  that  our  Microscopes   are  being  selected  by 

the  best  institutions,    after  critical  comparative  tests,  is 

the  best  of  evidence  as  to  the  quality  of  the  stands  and  the 

objectives. 

No.  45D. — With  2  oculars,  double  nosepiece, 
16mm  and  4mm  objectives,  with  upper  iris 
diaphragm 

No.  45F. — Same  as  above,  with  condenser  and 
lower  iris  diaphragm 

yVo.  45H. — With  2  oculars,  triple  nosepiece, 
16mm,  4mm  and  2mm,  oil  immersion  objec- 
tives, condenser  and  iris  diaphragms^  .  .  $72.50 


NET 


$39.50 


$47.00 


Spencer  Lens  Co.,  Buffalo,  N.  Y. 


HAU8AUER-JONE8   PRINTING  CO.,  BOFFAIO, 


LOCKED 
CASE 

BIOLOGY 
LIBRARY 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


